Multiple beam cathode ray tubes are frequently used to display alphanumeric and/or other visual pattern information. Such tubes have greater bandwidth than single beam tubes, which enables them to display more information at suitable brightness than the single beam type.
Typically, the multiple beam tubes utilize a plurality of electron beams which are arranged in an array. Accelerating means, focussing means and deflection means are disposed in or on the envelope of the cathode ray tube, and after being accelerated and focussed, the beams are deflected across the screen while repeatedly being turned on and off so as to form "dots" on the screen at respective scanning positions. In order to form the desired characters or other patterns, logic circuitry selectively controls each beam to be either on or off at each scanning position, and the resulting arrangement of "dots" forms the desired pattern.
The usual cathode-grid structure in such a multiple beam tube consists of a sheet cathode which emits electrons over its entire surface, and an array of grid elements disposed in front of the cathode. Each such grid element has a circular aperture therein for defining and passing an electron beam, and the apertures are collectively arranged in an array pattern which corresponds to the desired electron beam array pattern.
Such a cathode-grid structure has several inherent problems. Since each grid element must be individually controllable, a separate lead wire must be connected to each element. However, the leads to the respective elements must be kept distance from the apertures in adjacent grid elements as otherwise the electric fields around the leads will intermodulate adjacent electron beams. Further, since the spacing between adjacent grid elements is extremely small, typically given about 0.005", the leads cannot be run in these spaces, and since the entire grid may only be on the order of 1/10" on a side, appropriate positioning and connection of the leads is frequently extremely difficult.
Additionally, because of the above size considerations, the mechanical mounting of the grid elements is not easily accomplished. Since each element must be spaced from every other element, each must be separately supported in the tube. However, the space which is available for the mounting members may not be adequate for the effective and precise mounting which is required. Further, it should be noted that while these problems exist even when the array of electron beam sources is in a straight line pattern, they become more serious when a two dimensional source array, and a two dimensional grid array having a plurality of interior grid elements is employed.